ABSTRACT: Much literature in marine biology describes the extraordinary behaviour of sea urchins, e.g., Paracentrotus lividus, who cover their body with shells, stones and debris. The function of this strange behaviour, described as Œmasking¹, is
still a puzzle. Our experiment shows that sea urchins are loaded with more mussel shells when the delicate apical openings of their water vascular system which powers all their movements, are in danger of being occluded by floating sand. ŒMasking¹ shells
appear to function as an umbrella against floating particles.